This invention relates to cooking ovens, particularly those which are pyrolytically cleaned and more specifically relates to a novel control for such ovens to reduce oven cleaning time to that necessary to remove the existing food soil and to reduce the flow into the external atmosphere of unoxidized effluent from the pyrolytic cleaning process.
It is well known that smoke and other effluent is produced by cooking ovens during the cooking process and particularly during pyrolytic cleaning of the oven. Some effluent produced during cooking is desirable since it is the source of appetizing aromas. However, other cooking effluent and particularly all pyrolytic cleaning effluent, is undesirable. Many smoke eliminator systems are known to reduce effluent from the pyrolytic cleaning process. These provides an incidental benefit of reducing undesirable cooking effluent in some oven cleaning smoke elimination systems.
There are two principal approaches to automatic oven cleaning. The first is the continuous-clean system which employs a catalytic oven surface which promotes oxidation of food soil which drips and spatters on the oven walls during the cooking process. Due to the catalytic action, the food soil hydrocarbons are converted to carbon dioxide and water vapor with reasonable efficiency at temperatures near the maximum employed in the cooking processes which is about 500.degree. F. If high-temperature cooking processes, such as broiling, are employed with sufficient frequency and no oven soil reaches oven surfaces (such as windows) which are not coated with the catalytic material, the oven will remain reasonably clean. This system also tends to reduce both desirable and undesirable cooking process effluent discharge into the room atmosphere.
The second approach may employ oven cavities having porcelain enamel interior walls and a glass viewing window, all of which collect food soil during normal cooking use of the oven. To burn off this soil, the oven heater coils are operated to raise the temperature of the oven internal surfaces close to 900.degree. F., and usually to about 880.degree. F., for an extended period. The process produces a considerable amount of undesirable effluent which is passed through a known type of smoke eliminator, contained in the oven wall, and is discharged into the room atmosphere.
The effluent which is produced by the pyrolytic oven cleaning process will contain a high percentage of partial pyrolytic products which are relatively simple volatile hydrocarbons produced from the complex solid food soil hydrocarbons by elevated temperatures. The composition of the effluent is related to the temperature-time profile of the cleaning process and to the type of food soil being cleaned. In a typical cleaning cycle, the greatest volume of effluent is produced fairly early in the cleaning cycle as the oven temperature first passes through the 600.degree.-700.degree. F. region, as the temperature is increased to the approximately 880.degree. F. value which is maintained during most of the cycle. The remaining soil, however, must be treated for an extended period of time at the 880.degree. F. temperature to complete the final removal of the carbon-rich soil remaining after the more volatile partial pyrolytic products have been driven off.
The smoke eliminator which vents effluent to the room atmosphere is provided with a heater which causes the oven output duct to have a temperature higher than that inside the oven chamber. Thus, the smoke eliminator can further oxidize oven effluent before it reaches room atmosphere. Moreover, the heater in the smoke eliminator tends to direct effluent through that duct, rather than to other possible exit routes, such as around the oven door, and causes the effluent to oxidize rapidly to the carbon dioxide and water vapor end products that are more desirable than the untreated effluent. The smoke eliminator heater keeps the effluent in the smoke eliminator duct about 100.degree.-200.degree. F. hotter than the internal oven temperature, when there is no exothermic reaction in the smoke eliminator. When effluent burns in the smoke eliminator, the duct temperature may rise rapidly to 500.degree.-600.degree. F. above the oven temperature. This peaking of temperature in the smoke eliminator duct has been employed in the past as an indicator of the concentration of incompletely-oxidized components in the oven effluent and of the progress of the oven cleaning process. The correlation, however, has been found not good enough to provide a basis for satisfactory control, whereby the oven cleaning process can be terminated at a time related to the temperature peaking within the smoke eliminator duct.
As a result of the above uncertainties, present pyrolytically operated self-cleaning ovens require widely variable times, for example, from 1 to 4 hours, from the start of the cleaning cycle to completion of the conversion of all soil to either volatile material that escapes through the smoke eliminator or ash which is easily removed by wiping. The cleaning time can also vary within the same oven, operating from the same power source, because of different soil conditions, soil composition and prior bake-on history. Therefore, the user must estimate cleaning time requirements from past experience and observation of the oven soil conditions. In practice, this results in the use of much longer cleaning time settings than may be actually required. The user commonly learns and adopts the practice of always using a time setting equal to the longest time requirement that was ever encountered. Consequently, it is easily possible to waste two or more hours of cleaning energy on most cleaning runs that do not require maximum cleaning time. For a conventional oven, approximately 7 kilowatt hours would be wasted with this unnecessary extra cleaning.
Prior art pyrolytic cleaning systems are also subject to a condition known as smoke eliminator overload. Thus, the capacity of a smoke eliminator to convert undesirable partial pyrolytic effluent to a more desirable pyrolytic end product like carbon dioxide and water vapor is a function of temperature, oxygen availability and effluent dwell time in the smoke eliminator. In practical designs, it is possible for some oven soil conditions to produce effluent flow rates which exceed the smoke eliminator capacity and permit a substantial amount of effluent to pass through the smoke eliminator and into room atmosphere without being converted to pyrolytic end-products. The most serious cause of the smoke eliminator overload condition is that the smoke eliminator temperature is too low for effective oxidation at the time the first volatile effluent product arrives from the oven in the initial oven heat-up phase of the cleaning cycle. Since this first large influx of effluent tends to occur for oven temperatures in the 400.degree.-500.degree. F. range, the smoke eliminator duct temperature will then be in about the 500.degree.-700.degree. F. range. This is several hundred degrees too low for efficient oxidation of the effluent without a catalyst. This type of smoke eliminator overload early in the cleaning cycle is quite common because it can occur even when the oven is only lightly soiled.
In addition to the considerations given above, the time required to pyrolytically clean an oven varies widely with other parameters, e.g. oven manufacturing tolerances, such as the temperature limit switch deadband and the oven door gap. The temperature limit switch is necessary to keep maximum oven temperature below limits set to protect the temperature sensors and the porcelain oven lining and to prevent excessive external oven surface temperatures. These limits are only a few degrees above the minimum temperatures required for rapid pyrolyzation of carbon-rich food soil.